This invention relates to a method of tinting a mineral filler, and more particularly to increasing the whiteness of a mineral filler.
Mineral fillers, such as limestone, dolomite, kaolin, talc, feldspar, mica, barite, gypsum, diatomite and some clays are used to fill and/or extend industrial products, such as paint, paper, plastics, rubber, adhesives, caulks, drywall, joint cement, and athletic field markers. In a great number of such applications, the color of the mineral filler is of utmost importance, since the color of the mineral filler may affect the color of the finished product. Generally, white is the most desirable color, or the absence of color.
These mineral fillers come from mineral deposits, many of which are contaminated with various inorganic or organic elements or compounds that give the filler produced from these deposits a color or tint other than white.
Various attempts have been made to remove colorants from mineral fillers, with varying degrees of success. One particularly troublesome color contamination problem is the occurrence of a yellow tint (iron oxide) in limestone. Some of the yellow tint in mineral fillers, such as limestone, can be removed by bleaching. Differential froth flotation can separate some coloring minerals from the mineral filler. High intensity magnetic separation is also utilized to improve the brightness of some mineral fillers.
The processes for the improvement of the whiteness of certain industrial products, other than mineral fillers, are disclosed in the following U.S. Pat. Nos.:
______________________________________ 2,136,782 Bucy Nov. 15, 1938 2,191,947 Allen Feb. 27, 1940 2,950,207 Jones Aug. 23, 1960 3,669,896 Preininger et al Jun. 13, 1972 ______________________________________
The above Bucy and Allen patents are limited to the utilization of blueing agents with white pigments or with the industrial product, such as paints, varnishes, fabrics and paper, to prevent the yellowing of the products by aging. The Bucy patent is limited to a substantially colorless blueing pigment, specifically molybdenum trioxide, which gradually turns blue upon aging.
The Allen patent utilizes blueing agents, such as ultramarine blue or iron blues, for inhibiting the yellowing by aging of white baking enamels.
The Jones patent is limited to the use of a combination of a blue organic pigment and a violet organic pigment, both for eliminating the yellow tinge in filaments of regenerated cellulose.
The Preininger et al, patent relates only to the utilization of optical brighteners with inorganic white pigments.